1. Technical Field
The present invention relates to generate laser pulse trains using a Q-switch in a continuously pumped laser, particular to generate laser pulse trains in a continuously pumped laser using a Q-switch made by a quadratic electro-optic material and driven by a dual-polarity radio frequency (RF) control signal.
2. Technical Background
Among the methods to generate laser pulse trains, Q-switching is one of the most common schemes. In this scheme, the continuously pumped laser output is turned off by increasing the resonator loss (spoiling the resonator quality factor Q) periodically with the help of a modulated loss inside the resonant cavity. The Q-switching is a loss switching. Because the pump continues to deliver constant power at all time, energy is stored in the atoms in the form of an accumulated population inversion during the off (high-loss)-times. When the losses are reduced during the on-times, the large accumulated population is released, generating intense (usually short) pulses of light.
Several methods were used to Q-switch the laser pulses. A passive Q-switch, such as a saturable-absorber Q-switch (also known as a dye-cell Q-switch) uses some form of light absorbing material that saturates when the gain inside the cavity exceeds a certain level, at which time the laser begins oscillating. The dye then quickly drops below its saturation level and oscillation stops. The process automatically repeats to produce successive laser output pulses without the need for any external energy or control. Passive Q-switches are widely used because they are simple, but they have significant limitations, such as pulse-by-pulse amplitude fluctuation and no control over frequency of Q-switching or output pulse width.
The active Q-switches exhibit much stable performance. One example is an electro-optic (EO) Q-switch uses EO crystal, mainly KDP or LiNbO3 crystal which becomes birefringent when subjected to high electric voltage to create a cavity loss. Although this Q-switching technique is fast and precise, thus providing control over the peak output pulse width independent of output pulse frequency, it has some disadvantages. Because of the small EO coefficient associated with the materials, the radio frequency (RF) control voltage is high, usually in the kilovolts range, a very high power source and a mean of high voltage isolation are needed. It is difficult to generate laser pulse trains with very high pulse repetition frequency (PRF).
The other approach of Q-switching is an acousto-optic (AO) Q-switch. AO Q-switching techniques use an acousto-optic modulator to produce an RF acoustic wave in order to Bragg-diffract the light out of the cavity. This technique is simple and operates well at MHz PRF, but with the fixed or discrete frequency.
Although there have been many advances in the field of Q-switching, there is still a need for a Q-switching technique that has high PRF at desired frequency, low control voltage, compact size and low in cost.